Double and single ionization of hydrogenmolecules by fast-proton impact

Abstract

The ratio of double to single ionization of hydrogen moleculescaused by fast-proton impact was measured over a wide velocityrange (v = 4-24 au) using the coincidence time-of-flighttechnique. The value of this ratio for hydrogen molecules at thehigh-velocity limit was determined to be 0.18+0.01-0.02using the q/v dependence suggested by McGuire. Thisratio is smaller by about a factor of 1.8 for hydrogen than forhelium over the measured energy range and by about 1.4 at thehigh-velocity limit. This difference between the two targets isdue mainly to the single-ionization cross section, which wasmeasured to be larger by a factor of 1.79 ± 0.05 forhydrogen molecules than for helium. The double-ionization crosssection, in contrast, is similar for both helium and hydrogentargets. It is suggested that single ionization of hydrogenmolecules is more likely due to its smaller binding energy whilethe stronger electron-electron interaction in helium compensatesfor the smaller probability of proton impact ionization andleads to roughly equal double ionization of both targets. Forboth hydrogen and helium targets, the double- to single-ionization ratio is smaller for proton impact than for equal-velocity electron impact over the measured velocity range.